For more decorative starfish, coral, and shells, try Pottery Barn. They are selling summer decor for 50% off online and in-stores. These are larger, not really for favors though. I could definitely see them being used as part of center pieces or other decor. And of course, they are nice enough to decorate your home with too!

They are available in the resin material and metal. (I saw the metal ones in store and other assorted shells too).